To improve thermal efficiency of gasoline internal combustion engines, dilute combustion—using either air or re-circulated exhaust gas—is known to give enhanced thermal efficiency and low NOx emissions. However, there is a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include 1) improving ignitability of the mixture by enhancing ignition and fuel preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the engine under controlled auto-ignition combustion.
The controlled auto-ignition process is sometimes called the Homogeneous Charge Compression Ignition (HCCI) process. In this process, a mixture of combusted gases, air and fuel is created and auto-ignited simultaneously from many ignition sites within the mixture during compression, resulting in very stable power output and high thermal efficiency. Since the combustion is highly diluted and uniformly distributed throughout the charge, the burned gas temperature, and hence NOx emission, is substantially lower than that of the traditional spark ignition engine based on propagating flame front, and the diesel engine based on an attached diffusion flame. In both spark ignition and diesel engines, the burned gas temperature is highly heterogeneous within the mixture with very high local temperature creating high NOx emissions.
Engines operating under controlled auto-ignition combustion have been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burned gases remaining from the previous cycle, i.e. the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture. In four-stroke engines with traditional valve means, the residual content is low, controlled auto-ignition at part load is difficult to achieve. Known methods to induce controlled auto-ignition at part load include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with fuel that has wider auto-ignition ranges than gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
Engines operating under controlled auto-ignition combustion have been demonstrated in four-stroke gasoline engines using variable valve actuation to obtain the necessary conditions for auto-ignition in a highly diluted mixture. Various fueling controls including split and single injections have been proposed for use in conjunction with valve control strategies to maintain stable auto-ignition combustion across a variety of engine load conditions.
In commonly assigned U.S. patent application Ser. No. 10/899,457 an exemplary fuel injection and valve strategy for stable, extended controlled auto-ignition is disclosed. Therein, during operation at low load, a first injection of fixed amount of fuel during the negative valve overlap period is followed by a second fuel injection during the subsequent compression stroke. The injection timing for the first injection retards and the injection timing for the second injection advances in a continuous manner as the engine load increases. During operation with intermediate part load, a first injection of fuel during the negative valve overlap period followed immediately by a second injection of fuel during the subsequent intake stroke supports auto-ignition. Optimal separation of the two injections is around 30 to 60 degrees crank angle. The injection timings of both injections retard in a continuous manner as the engine load increases. And, during operation with high part load, a single fuel injection during the intake stroke supports auto-ignition. The injection timing retards as the engine load increases.
While the advances outlined above have successfully demonstrated controlled auto-ignition capabilities at steady state conditions, rapid load changes or transients may introduce undesirable combustion results.